Deck 12: Moving Proteins Into Membranes and Organelles

Question 1

Which of the following is true about ER import?

A)N-terminal signal sequences have been determined to be necessary for ER import because if they are added to sequences which are not normally targeted to the ER,they will end up in the ER.
B)Signal recognition particles (SRPs)are needed during co-translational import.
C)Ribosomal translation of a 6-12 amino acid sequence on the N-terminus will initiate the process through interactions with SRP.
D)GTP hydrolysis by the Sec61 translocon causes ribosomal translation to begin again after it is halted by SRP binding.

Answer 1

C

Question 2

In a cell-free protein synthesis system utilizing microsomes from fragmented ER,under which condition could you determine if the new protein was imported into the microsome?

A)Ribosomes and mRNA are incubated with microsomes,then a protease is added and the results are analyzed.
B)Ribosomes and mRNA are incubated with microsomes,then a protease and detergent are added and the results are analyzed.
C)Ribosomes and mRNA are incubated with protease,then microsomes are added and the results are analyzed.
D)Ribosomes and mRNA are incubated with microsomes,then detergent is added and the results are analyzed.

Answer 2

A

Question 3

What are the general features of an N-terminal signal sequence that targets secretory proteins to the ER?

Answer 3

N-terminal signal sequences targeting proteins to the ER are 16 to 30 amino acids in length and have a hydrophobic core of 6 to 12 amino acids.Preceding the core is one or more positively charged amino acids.Otherwise,N-terminal signal sequences have little in common.

Question 4

Protein insertion into the mammalian ER membrane is typically:

A)cotranslational.
B)post-translational.
C)pretranslational.
D)quasitranslational.

Question 5

In the absence of targeting information,what is the default location of proteins synthesized on cytosolic ribosomes?

Question 6

What is the meaning of "quality control in the ER?"

Question 7

GPI-anchoring serves a special function,especially in polarized epithelial cells,because this modification serves to target proteins to the:

A)RER.
B)Golgi.
C)plasma membrane.
D)nucleus.

Question 8

Type I membrane proteins have all of the following properties,except:

A)cleavable signal sequence.
B)internal signal-anchor sequence.
C)internal stop-transfer sequence.
D)N-out,C-in topology.

Question 9

Having misfolded soluble or secretory proteins in the RER contributes to what investigators call the "traffic jam," a scenario associated with a number of human diseases where the normal transport of proteins is blocked by these abnormal proteins and the inability of protein complexes to arrive at their correct site and function properly.Briefly describe how the cell overcomes this particular traffic jam by exporting the misfolded proteins out of the RER into the cytosol,where they are degraded by the proteasome.

Question 10

Post-translational translocation of some secretory proteins in yeast is powered by:

A)ATP hydrolysis by BiP.
B)cAMP hydrolysis by cAMP phosphodiesterase.
C)GTP hydrolysis EF-Tu.
D)phospholipid hydrolysis by phospholipase C.

Question 11

Why are bacteria often a poor choice for the production of proteins for therapeutic purposes?

Question 12

In multipass membrane proteins synthesized in association with membrane-bounded ribosomes of the rough ER,signal-anchor and stop-transfer anchor sequences alternate.What do these sequences do?

Question 13

Unassembled or misfolded proteins in the RER can be damaging to the physiology of a cell and therefore are transported to the cytosol where they are degraded.This transport process is referred to as:

A)polyubiquitination.
B)disulfide isomerization.
C)dislocation.
D)O-linked glycosylation.

Question 14

A transmembrane receptor that functions at the cell membrane has an exoplasmic N-terminal sequence,a signal-anchor sequence,and a stop-transfer-anchor sequence.This protein was first inserted into the membrane where?

A)at the plasma membrane
B)in the cis-Golgi
C)in the late endosome
D)in the ER

Question 15

The topology of membrane proteins can often be predicted by computer programs that identify_____ topogenic segments.

A)glycosylation-specific
B)hydrophilic
C)hydrophobic
D)cleavable signal sequence

Question 16

All the following proteins interact with exposed amino acids during protein folding in the ER,except:

A)BiP.
B)calnexin.
C)PDI.
D)prolyl isomerase.

Question 17

Sorting of proteins to mitochondria and chloroplasts is:

A)cotranslational.
B)post-translational.
C)pretranslational.
D)quasitranslational.

Question 18

Which of the following proteins involved in cotranslational translocation of proteins into the ER membrane is NOT a GTP-hydrolyzing protein?

A)α subunit of the SRP receptor
B)elongation factors in ribosome-mediated mRNA translation
C)P54 subunit of SRP
D)Sec61 translocon

Question 19

Glycosylation,a post-translational modification of proteins,occurs in the:

A)Golgi.
B)proteasome.
C)mitochondria.
D)none of the above

Question 20

Tom/Tim and Toc/Tic protein complexes are involved in:

A)post-receptor recognition events in the cytosolic folding of proteins prior to import into mitochondria or chloroplasts.
B)pre-proteasomal steps in tagging aged proteins for degradation.
C)protein translocation into mitochondria and chloroplasts,respectively.
D)resetting biological clocks following rounds of intense protein synthesis.

Question 21

Which of the following components of mitochondrial import are NOT required for a sequence containing a matrix-targeting sequence and an intermembrane-space-targeting sequence?

A)Tom
B)Tim
C)Oxa1
D)membrane-bound protease

Question 22

Which of the following is present in the nuclear export sequence of PKI (an inhibitor of protein kinase A)?

A)a proline-rich sequence
B)a leucine-rich sequence
C)a lysine-rich sequence
D)all of the above

Question 23

PTS1- and PTS2-bearing matrix proteins are targeted to:

A)a common cytosolic receptor.
B)a common import receptor and translocation machinery on the peroxisomal membrane.
C)a common receptor on the nuclear pore that catalyzes entry into the nucleus via pore targeting sequences.
D)a common receptor protein within the peroxisomal matrix that activates protein processing for PTS1- and PTS2-bearing proteins.

Question 24

The nuclear pore complex allows for:

A)passive diffusion of smaller molecules.
B)import of proteins.
C)active transport of very large molecules.
D)all of the above

Question 25

A polypeptide chain contains an amphipathic helix,with arginine and lysine residues on one side and hydrophobic residues on the other.It will likely enter:

A)the peroxisome.
B)the ER.
C)the lysosome.
D)the mitochondria.

Question 26

How does Ran·GTP participate in the nuclear export of the HIV Rev protein?

Question 27

Protein import into the mitochondrial matrix is supported by energy input from:

A)ATP hydrolysis by chaperone proteins in the cytosol.
B)ATP hydrolysis by chaperone proteins in the mitochondrial matrix.
C)the proton-motive force across the inner mitochondrial membrane.
D)all of the above

Question 28

Which type of RNA participates in nuclear export of mRNA?

A)snRNA
B)hnRNA
C)tRNA
D)rRNA

Question 29

The nuclear pore complex (NPC)contains_____ structures that form a gel-like matrix that allow small molecules to diffuse through,but require larger proteins to enter via importin or other nuclear chaperones.

A)nuclear localization signals (NLS)
B)nuclear lamina
C)structural nucleoporin
D)FG-nucleoporin

Question 30

Many peroxisomal matrix proteins are imported as:

A)folded proteins.
B)nascent chains in the process of completing their elongation.
C)protein fragments that are spliced together within the peroxisome.
D)unfolded proteins.

Question 31

To what extent do peroxisomal matrix protein import and peroxisomal membrane protein import share the same machinery?

Question 32

In a cell that lacks cytosolic Hsc70:

A)import of proteins into the mitochondrial matrix would be diminished.
B)proteins in the ER would not fold properly.
C)the peroxisome would contain more catalase.
D)generation of ATP from the electron transport chain would happen at the plasma membrane.

Question 33

Protein sequences for targeting to mitochondria or chloroplasts are located at:

A)the C-terminus of the precursor protein.
B)amino acid position 173 in most mitochondrial and chloroplast proteins.
C)the N-terminus of the precursor protein.
D)the second and third answers are correct

Question 34

Transport of unspliced HIV mRNA from the nucleus to the cytoplasm of host cells is promoted by a virus-encoded protein named:

A)Tat.
B)Rev.
C)nucleoplasmin.
D)Ran.

Question 35

Many peroxisomal matrix proteins are imported as:

A)folded proteins.
B)nascent chains in the process of completing their elongation.
C)protein fragments that are spliced together within the peroxisome.
D)unfolded proteins.

Question 36

Unlike mitochondria and chloroplasts,peroxisomes can arise _____ from precursor membranes,as well as by division of preexisting organelles.

A)as condensate
B)as dispersions
C)de novo
D)as vaporware

Question 37

What is meant by de novo formation of peroxisomes?

Question 38

During in vitro translation of mitochondrially targeted proteins,when must mitochondria be added for import of proteins synthesized on cytosolic ribosomes?

Question 39

How are proteins imported into the thylakoids of chloroplasts?

Question 40

During the import of proteins into the nucleus,the α importin subunit binds directly to:

A)FG nucleoporins.
B)Ran·GDP
C)basic nuclear localization signals in cargo proteins.
D)all of the above

Question 41

During the process of nuclear import,a GEF works in the:

A)cytoplasm to exchange GTP for GDP bound to Ran.
B)cytoplasm to use GTP to release Ran from importin.
C)nucleus to exchange GTP for GDP bound to Ran.
D)nucleus to activate the intrinsic GTPase activity of Ran.